1. Field of the Invention
The present invention relates generally to magnetic heads. In particular, it relates to a dual magnetic head for recording both a video signal on a main track and an audio signal on the guard band between the main tracks.
2. Background Art
When recording data on a magnetic disk, such as a floppy disk, the information is recorded on circular tracks during rotation of the floppy disk. Furthermore, a guard band is generally provided between two adjacent tracks so as to absorb any tracking error due to expansion and contraction of the floppy disk occurring with variations of temperature and humidity or due to the positioning error arising from the inaccuracy of the head-tracking mechanism.
Various types of recording apparatus for a floppy disk have been developed in the past. A well known one of these is a still picture recording apparatus, particularly a so-called electronic camera, in which an image of an object taken through a lens is converted into an electronic signal which is in turn recorded on the floppy disk. Such a floppy disk is called a video floppy. For reproduction, there is known an apparatus which rotates the floppy disk and reproduces the image signal as a still picture on a display device such as television picture screen, where the image signal for one field is recorded on one circular track. A method of reproducing a still picture with the one-field image signal is referred to as field recording and reproduction. On the other hand, an alternative method records each of two fields of a frame on two tracks and reproduces a still picture on the basis of one frame produced by the image signal on the two tracks. This method is designated as a frame recording and reproduction method. According to the standard for electronic still cameras, the floppy disk is rotated at a speed of 3600 rpm and has tracks with widths of 60 micrometers which are concentrically formed at intervals of 100-micrometer pitch. The nominal width of the guard band is 40 micrometers, and the tolerance of the track radius for the expansion and contraction of the floppy disk and the tracking accuracy error is .+-.4 micrometers.
On the other hand, in the case of displaying a still picture on a television picture screen or the like, an audio signal including data such as voice and music data recorded in correspondence with an image signal is generally reproduced simultaneously with the video signal. Techniques of recording such information in the guard band are proposed in Japanese Patent Provisional Publication No. 61-129701 and Japanese Patent Provisional Publication No. 61-153803. More specifically, conventional tracks are used as main tracks for recording of the image signal and a data signal such as a voice is time-compressed and recorded on a guard band between the main tracks. The data signal is recorded at an azimuth with the main tracks and the guard bands. The information signal recorded by way of time-compression is time-restored after being reproduced. A magnetic head (main head) for recording an image signal on the main track and a magnetic head (data head) for recording a data signal on the guard band may be separate from each other or constructed integrally as shown in Japanese Patent Provisional Publication No. 61-129701.
The data signal including data such as voice, after the corresponding image signal is recorded on a main track, is recorded on a guard band adjacent to the main track.
As described above and illustrated in FIG. 12, recording is performed such that, for example, a main track 2 has a width of 60 micrometers and a guard band 4 has a width of 40 micrometers. However, in two extreme cases, the guard band is effectively reduced to a minimum width. In the first case, a floppy disk 1 containing the main tracks 2 and the guard band 4 contracts to the maximum allowable extent and the tracking accuracy greatly deteriorates in one direction when an image signal is recorded on one main track. In the second case, the floppy disk 1 expands to the maximum allowable extent, and, the tracking accuracy also greatly deteriorates, but in the opposed direction when an image signal is recorded on the next main track 2. Because of the allowed expansion and contraction, a non-recorded portion to be used as the guard band 4 has a width of 40-(14.times.2) micrometers, i.e., 12 micrometers. Consequently, when data is recorded on the guard band 4 by means of the data head arranged to form a track with a width of 40 micrometers, the data track overlaps by 14 micrometers with each of both the main tracks 2.
In such a case, not only the data recording made by the data head becomes unsatisfactory, but also the signal on the overlapped portions 3 of the main tracks 2 is degraded.